Pipes made of plastics material are used extensively in industry and in construction. Many applications now use pipes made from plastics materials such as polyethylene and polypropylene rather than metal. One such application is the pipework used in petroleum installations such as garage forecourts.
In the design of petroleum forecourt installations for example, it is regarded as increasingly important to contain and detect any leaks of petroleum or diesel fuel from subterranean pipes which connect one or more storage tanks to dispensing pumps in the installation. To that end, many current designs of forecourt installation utilise secondary containment. This involves containing each supply pipeline in a respective secondary containment pipeline which is optionally sealed at its ends to the fuel supply pipelines. The secondary containment pipeline prevents leaks from the fuel supply pipeline from being discharged into the environment, and also can convey leaked petrol to a remote sensing device.
Whilst every effort is made to avoid having joints in an underground supply pipeline, other than inside manhole chambers, these joints sometimes cannot be avoided. Such joints are conventionally made using special fittings and the connections are made using a chemical-based jointing compound or by electrofusion welding. The latter technique is preferred in many applications.
In conventional single containment plastic piping systems, successive lengths of plastic pipe are joined end to end using so-called electrofusion couplings or welding muffs, which typically comprise short plastic sleeves providing sockets at either end having internal diameters of a size to receive the ends of the respective pipes as a close fit and incorporating electrical resistance heating windings. Thus two adjoining pipe lengths can be connected end to end by inserting the adjoining pipe ends into such an electrofusion coupler from opposite ends thereafter passing electric current through the heating windings in order to fuse the internal surfaces of the electrofusion coupling and the adjoining the external surfaces of the inserted pipe ends, thereby welding the pipe ends to the electrofusion coupling to form a fluid tight joint.
Hitherto, in constructing secondary containment pipeline systems utilising plastic pipes, the pipe lengths forming the inner plastic pipe have been connected end to end by a first series of electrofusion couplings, whilst the pipe lengths forming the outer pipe have been connected end to end by a second series of electrofusion couplings distinct from and separate from those connecting the lengths of inner pipe. This procedure inevitably involves threading the outer pipe and the electrofusion couplings for the outer pipes over the inner pipes and/or the longitudinal displacement of the outer pipes and the associated electrofusion couplings relative to the inner pipes. This involves substantially more labour than the construction of a comparable single containment pipe system by similar techniques. In addition, it is normal practice to complete the primary pipework and test its integrity prior to completing the secondary containment system. This inevitably means multiple handling for the couplings. For example, couplings for the secondary pipes have to be slid into position in advance then moved aside as necessary. Alternatively, sections of secondary pipe and the associated couplings have to be fed over the length of the primary pipe once it has been assembled, welded and tested. This is both time consuming and labour intensive.
Matters are further complicated if a joint is being formed in the latest composite primary/secondary pipe. In this type of pipe, an example of which is described in GB9824955.6 and PCT/GB98/03422 (PetroTechnik Ltd), the primary and secondary pipe are formed as one. Conventional welding sockets cannot be used at all to weld this type of pipe because there is insufficient space between primary and secondary pipes to accommodate the inevitable terminal pins on the welding socket used on the primary pipe. Furthermore, the elbows and Tee-junctions for this type of pipe have to have a different, larger diameter than the pipe itself. This is because couplings for the primary pipe-to-primary elbow connection have to be accommodated within the secondary pipe-to-outer elbow connection. This requires a wide range of couplings to be made where the opposing diameters of items to be joined differ according to the intended use.
Accordingly, it is an object of the present invention to overcome or at least mitigate one or more of the problems outlined above.